The prior art has long sought an effective method of preventing the oxidation of metal objects which are exposed to an oxidizing environment. However, the methods and apparatus of the prior art have proven to be relatively ineffective.
Generally stated, the problem addressed by the present invention arises because objects made of metal are frequently exposed to oxidizing environments. These oxidizing environments contain one or more chemical substances which, under the relevant conditions, tend to be reduced.
In an oxidizing environment, metal objects tend to give up electrons, thereby reducing the substances in the surrounding environment, and oxidizing the surface of the metal object. As the oxidation progresses, the metal object eventually becomes degraded to the point that it is unsuitable for its intended purpose.
Examples of the problem include the metal fenders of land vehicles, and the metal girders of vehicular bridges, which are exposed to salt that is spread on the roads to prevent the formation of ice in cold climates. The salt melts the snow or ice and produces an aqueous salt solution. A number of the substances in the solution have a sufficient reduction potential so as to extract electrons from the surfaces of the metal fenders and girders, thereby oxidizing the metal. If the fenders or girders are made of iron or steel, then the oxidation may first produce the undesirable appearance of rust on the exterior surface. If the oxidation of the fender is allowed to continue, then the fender will rust through at various locations, and then disintegrate. Similarly, if the girder of a vehicular bridge is allowed to oxidize for a sufficiently long period of time, it becomes unable to carry the necessary load and collapses.
Sea water also presents an oxidizing environment for the hulls of ships and boats which are made of metal, as well as offshore oil wells and the like. Once again, if the oxidation is allowed to continue, the structure eventually collapses or disintegrates.
In response to this problem, numerous methods have been devised to reduce the rate of oxidation of metal objects. The most common method is to apply a protective coating to the surface of the metal before it is placed into operation. However, the coating eventually degrades and exposes the metal to the oxidizing environment. The results in the necessity of repeating the coating operation, or replacing the metal object (both of which can be impractical and relatively expensive).
The prior art also includes numerous cathodic protection systems. Generally speaking, these systems treat the metal object to be protected from oxidation as the cathode of an electrolysis circuit. These methods normally require an anode, a source of electric energy, and an aqueous solution. The anode and cathode must be in contact with the aqueous solution. The source of electric energy is then used to create a current between the anode and the cathode. As the source of electric energy provides electrons to the cathode (which is the metal object being protected from oxidation), the substances in the aqueous solution that have sufficient reduction potential to be reduced acquire the electrons provided by the electric current, rather than electrons from the metal, and are reduced. The rate of oxidation of the cathode (the metal object being protected from oxidation) is significantly reduced because the majority of the electrons needed for reduction of the chemical substances in the aqueous solution (the environment surrounding the metal object being protected) are provided by the electric current, rather than the metal in the cathode.
U.S. Pat. No. 3,242,064 discloses a cathodic protection system. It provides a corrosion reduction system in which pulses of direct current (DC) are supplied to the metal surface to be protected, such as the hull of a ship. The environment surrounding the hull varies, thereby varying the amount of current necessary to prevent oxidation of the metal hull. A signal from a sensing half-cell is used to automatically control the ratio of the pulse duration to the time between the pulses, depending on the conditions.
U.S. Pat. No. 3,692,650 also discloses a cathodic protection system. It is applicable to well casings and pipelines that are buried in conductive soils, the inner surfaces of tanks which contain corrosive solutions, and the submerged portions of ship hulls, pier structures and other offshore metal structures. This system uses a short pulsed DC voltage, and a continuous direct current. The width of the voltage pulses is sufficient to permit acid ion conversion, but not wide enough to permit undesirable chemical reactions. The pulse repetition frequency is made equal to the resonant frequency of the series circuit of the capacitance of the taffel double layer (a layer of charge that is allegedly formed at approximately 100 Angstroms from the surface of the structure) and the inductance between the anode and the cathode structure. The voltage amplitude is selected to give a maximum throw down the structure in order to effect polarization as quickly as possible. Throw is defined as the distance from the point at which current is supplied to the structure, to the point at which the current shorts back to the anode.
The cathodic protection systems of the prior art have failed to achieve an effective process of preventing the oxidation of metal objects. Moreover, they are of very limited utility with respect to metal objects that are not at least partially immerged in an electrically conductive medium, such as sea water or a conductive soil. Accordingly, above ground metal objects (such the as metal fenders of land vehicles, and the metal girders of vehicular bridges) are not effectively protected by these systems, because they are not regularly immerged in the electrolyte solution which is required to complete the series circuit between the anode and the cathode.